Spatially chirped pulses for femtosecond laser ablation through transparent materials

ABSTRACT

Temporal focusing of spatially chirped femtosecond laser pulses overcomes previous limitations for ablating high aspect ratio features with low numerical aperture (NA) beams. Simultaneous spatial and temporal focusing reduces nonlinear interactions, such as self-focusing, prior to the focal plane so that deep (˜1 mm) features with parallel sidewalls are ablated at high material removal rates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional Application Nos.61/319,757 and 61/384,956, filed Mar. 31, 2010 and Sep. 21, 2010,respectively, the entire disclosures of which are hereby incorporatedherein by reference.

This invention was supported, in part, using funds provided by the termsof EB003832 and MH085499 awarded by the National Institutes of Healthand/or the terms of FA9550-07-10026 and FA9550-10-C-0017 awarded by theAFOSR and/or by the terms of 09BGF-03 and 09BGF-04 awarded by theColorado Bioscience Discovery. The government has certain rights to thisinvention.

FIELD OF THE DISCLOSURE

The disclosure relates to optics and specifically to mechanisms forfocusing spatially chirped pulses for femtosecond laser materialmodification.

BACKGROUND

Micromachining with femtosecond laser pulses, in which the transientgeneration of a plasma leads to the ablation of material, is a powerfultechnique to cut chemically inert media such as glass. This procedureuniquely facilitates the prototyping of three-dimensional (3D)microanalytic devices with sub-diffraction-limited features. However,single-step processing has been limited in the size and aspect ratio ofthe features that can reasonably be produced in these media. Asexamples, in most fabrication techniques a laser beam is focused on thefront surface of the substrate and ablation proceeds from the top down.Thus, successive pulses must focus through debris created by earlierpulses, and the pulses ultimately interact with the walls of thestructure as the feature becomes deeper. This leads to a tapering of thefeature that limits the aspect ratio.

An improved machining method would enable processing to take placethrough the backside of the wafer. Machining in this manner means thatsuccessive pulses would no longer focus through debris, nor interactwith the walls, and thus makes it possible to produce exceptionally highaspect ratio features. Some systems have been designed which ablatedhigh aspect ratio structures on the back surface of 1 mm thick glass athigh numerical aperture (0.55 NA). The working distance of theseoriginal systems has been extended by employing a long working distanceobjective at 0.42 NA.

Other systems have achieved high aspect ratio structures with Besselbeams by focusing on the backside of the substrate. However, Besselbeams do not have the same 3D control as other techniques.

An improvement to backside machining would be to use lower NA beams toincrease the interaction volume but without compromising 3D spatialconfinement.

SUMMARY

A high rate of cutting is important for applications where a significantvolume of material must be ablated. For example, microfluidic devicesrequire networks of channels that extend centimeters in length and lasersurgery involves the removal of many cubic millimeters of material. Ahigh rate of cutting appropriate for such applications is achieved bythe introduction of temporal focusing.

In accordance with at least some embodiments of the present disclosure,in this technique of temporal focusing, spatial chirping is used to forma frequency-distributed array of low NA beamlets, which coalesce toreform a transform-limited and diffraction-limited pulse at the focus ofthe objective.

By adapting temporal focusing to an ablation beam, embodiments of thepresent disclosure are able to improve the machining rate and performselective ablation through thick, optically transparent samples at 0.05NA, for example.

As a non-limiting example, temporally focused beams can be employed towrite approximately 2 mm long, sub-surface microfluidic channels in amicrofluidic device while simultaneously improving channel shape.Similar gains can also be envisioned for micromachining applications andother applications where materials are cut, bonded, modified (e.g.,locally densified), etc.

In accordance with at least some embodiments of the present disclosure,an optical system is provided that comprises:

a first optical element or collection of optical elements whichspatially chirp and collimate temporally chirped pulses of light as wellas cause intensities of a plurality of frequencies of the spatiallychirped and collimated light to overlap in time but not overlapspatially; and

a second optical element or collection of optical elements which focusthe plurality of frequencies to overlap spatially at a focal plane.

A method is also provided which generally comprises:

generating temporally chirped pulses of electromagnetic light;

spatially chirping and collimating the temporally chirped pulses oflight;

causing frequencies of the spatially chirped and collimated light tooverlap in time; and

causing the frequencies of the spatially chirped and collimated light toonly overlap spatially at a focal plane.

It should be noted that there are a number of different kinds of spatialchirp. One type of spatial chirp is a transverse spatial chirp where thewavelength components of a beam are all parallel to one another beforebeing focused. Another type of spatial chirp is an angular spatialchirp. It should be noted that while embodiments of the presentdisclosure are primarily described in connection with a transversespatial chirp, the disclosure is not so limited and those of ordinaryskill in the art will appreciate that certain optical elements describedherein can be modified to accommodate the use of angular spatial chirpswithout departing from the scope of the present disclosure.

The Summary is neither intended or should it be construed as beingrepresentative of the full extent and scope of the present invention.The present disclosure is set forth in various levels of detail and theSummary as well as in the attached drawings and in the detaileddescription of the disclosure and no limitation as to the scope of thepresent disclosure is intended by either the inclusion or non inclusionof elements, components, etc. in the Summary. Additional aspects of thepresent disclosure will become more readily apparent from the detaileddescription, particularly when taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The present disclosure is described in conjunction with the appendedfigures:

FIG. 1 is a schematic diagram depicting an optical system in accordancewith embodiments of the present disclosure;

FIG. 2 is a schematic diagram of a sample mounted in a chamber inaccordance with embodiments of the present disclosure;

FIG. 3A is a side view of a geometric optics model in accordance withembodiments of the present disclosure;

FIG. 3B is a chart depicting the shape of the beam spot created in FIG.3A;

FIG. 3C is an image of the focal plasma in air in accordance withembodiments of the present disclosure;

FIG. 3D is a chart depicting an asymmetric beam spot at focus with theaddition of fused silica to simulate backside machining in accordancewith embodiments of the present disclosure;

FIG. 4A is a chart depicting the spatio-temporal beam propagationsimulated in the spatially chirped dimension, x, generated by Fourierpulse propagation using a non-paraxial propagator where z=0 inaccordance with embodiments of the present disclosure;

FIG. 4B is a chart depicting the spatio-temporal beam propagationsimulated in the spatially chirped dimension, x, generated by Fourierpulse propagation using a non-paraxial propagator where z=0.5 f inaccordance with embodiments of the present disclosure;

FIG. 4C is a chart depicting the spatio-temporal beam propagationsimulated in the spatially chirped dimension, x, generated by Fourierpulse propagation using a non-paraxial propagator where z=0.8 f inaccordance with embodiments of the present disclosure;

FIG. 4D is a chart depicting the spatio-temporal beam propagationsimulated in the spatially chirped dimension, x, generated by Fourierpulse propagation using a non-paraxial propagator where z=0.9 f inaccordance with embodiments of the present disclosure;

FIG. 4E is a chart depicting the spatio-temporal beam propagationsimulated in the spatially chirped dimension, x, generated by Fourierpulse propagation using a non-paraxial propagator where z=f inaccordance with embodiments of the present disclosure;

FIG. 5 is a chart depicting the depth of focus and the B-integral as afunction of beam aspect ratio in accordance with embodiments of thepresent disclosure;

FIG. 6 is a cross-sectional view of a sample that compares the shapes ofchannels fabricated by chemical etching, front surface laser ablation,and back surface laser ablation;

FIG. 7 is a block diagram depicting a first optical system in accordancewith embodiments of the present disclosure;

FIG. 8 is a block diagram depicting a second optical system inaccordance with embodiments of the present disclosure; and

FIG. 9 is a cross-sectional view of a bundle of fiber optic elementsused to carry different wavelengths of a spatially-chirped beam of lightin accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

The ensuing description provides embodiments only, and is not intendedto limit the scope, applicability, or configuration of the claims.Rather, the ensuing description will provide those skilled in the artwith an enabling description for implementing the described embodiments.It being understood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope ofthe appended claims.

Specifically, mechanisms are provided in which temporal focusing ofspatially chirped femtosecond laser pulses overcome previous limitationsfor ablating high aspect ratio features with low numerical aperture (NA)beams (e.g., beams having NA of approximately 0.03-0.05). Simultaneousspatial and temporal focusing reduces nonlinear interactions, such asself-focusing, prior to the focal plane so that deep (e.g.,approximately 1 mm) features with parallel sidewalls are ablated at highmaterial removal rates (e.g., 25 μm3 per 80 μJ pulse) at 0.03-0.05 NA.

This technique can be applied to the fabrication of microfluidic devices(constructed of glass, similar silica-based materials,Polydimethylsiloxane (PDMS), or the like for instance) by ablationthrough the back surface of thick (at least 6 mm) fused silicasubstrates. It may also be used to ablate other materials such asplastics, glass, composites of glass and plastic, metal, ceramics, andthe like as well as bone under aqueous immersion to producecraniotomies.

In other words, several embodiments of the present disclosure will bedescribed in connection with a system designed to ablate channels and/orholes on the back surface of a material or sample. It should beappreciated, however, that embodiments of the present disclosure are notso limited. Rather, the concepts disclosed herein can be applied to cutmaterials, bond materials, modify materials (e.g., bend, densify, etc.),and so on.

As one non-limiting example, embodiments of the present disclosure canbe utilized in surgical applications (e.g., orthopedic surgicalapplications, opthalmic surgical applications, orthoscopic surgicalapplications, etc.).

As another non-limiting example, embodiments of the present disclosurecan be utilized in data storage applications by writing logical bitsonto a disk drive or array of disk drives, which may be stacked on oneanother to conserve space. Since the spatially chirped pulses of lightcan be focused through materials, the disks may be stacked one on top ofanother and data may be written to any one of the disks in the stack.The bits of data may be written to the disks by simply alteringproperties of the disk material (e.g., densifying, activating alight-response material on the disk, etc.) rather than actually cuttingthe material.

As an example, when 50 μJ, 60 fs, pulses (centered at 800 nm) arefocused through a 6 mm thick fused silica sample at 0.05 NA withouttemporal focusing, it can be observed that the beam self-focuses andcollapses into a filament. This self-focusing makes it difficult orimpossible to selectively ablate the back surface of the sample, and theentire thickness of the glass is modified along the direction ofpropagation. Also, without spatially chirped pulses, self-focusing andsupercontinuum generation result in a loss of intensity at the focus.Conversely, with the temporal focusing proposed by the presentdisclosure, by using the same pulse energy and duration and focal spotsize, it becomes possible to selectively ablate only the back surface ofthe sample leaving the glass volume nearly unblemished. Moreover, withspatially chirped pulses, self-focusing and continuum generation aresuppressed and the backside of the glass sample is ablated with greatprecision.

Referring initially to FIG. 1, an optical system 100 which enablestemporal focusing will be described in accordance with at least someembodiments of the present disclosure. The optical system 100 maycomprise an input light 104 that is provided as an input to a firstoptical element or set of optical elements 108. The first opticalelement or set of optical elements 108 conditions the input light 104and generates a first output light 116. The first output light 116 isthen provided to a second optical element or set of optical elements120, which focus the first output light at a specific focal point, whichmay be located on a sample 124.

In some embodiments, the first optical element or set of opticalelements 108 comprise a single-pass, double grating configuration tospatially chirp the input light 104 (e.g., remove the temporal chirp).The input light 104 may comprise one or a plurality of positivelychirped femtosecond pulses generated by, for example, a 1 kHz Ti:Al2O3chirped pulse amplification system. The pulses may be slightlypositively chirped to avoid pulse front tilt at the second opticalelement or set of optical elements 120, given the single-pass, doublegrating configuration used to spatially chirp the beam.

The first optical element or set of optical elements 108 may beconfigured to remove the temporal chirp from the input light 104. Wherethe first optical element or set of optical elements 108 comprises a setof optical elements, a plurality of individual optical elements 112 a-dmay be included in the first optical element or set of optical elements108. As one example, a first individual optical element 112 a maycorrespond to a mirror, a second individual optical element 112 b maycorrespond to a diffraction grating (or prism), a third individualoptical element 112 c may correspond to another diffraction grating (orprism), and a fourth individual optical element 112 may correspond toanother mirror. As a more specific non-limiting example, the firstoptical element or set of optical elements 108 may comprise a gratingsystem which includes two 600 l/mm gratings 112 b, 112 c used at anangle of incidence of 36 degrees and a separation of 630 mm, as measuredalong the perpendicular between the gratings. A suitable type of grating112 b, 112 c that may be used is manufactured by Thorlabs under partnumber GR25-0608.

The grating separation and angle can be selected to minimize second andthird order dispersion of the light as it travels through the firstoptical element or set of optical elements 108. Pulse compression can bethird-order limited as a result of the mismatch between the 1200 l/mmgratings in the laser compressor which generates the input light 104 andthe 600 l/mm gratings used to spatially chirp the beam. In someembodiments, the net efficiency of the two 600 l/mm gratings is 50%.

The first optical element or set of optical elements 108 conditions theinput light 104 to generate the first output light 116. In someembodiments, the radius of the first output beam of light 116 incidenton the second optical element or set of optical elements 120 (e.g., afocusing optic) is approximately 7.4 mm in the chirped dimension, asmeasured to the 1/e2 radius of the central intensity. In the unchirpeddimension, the beam 116 width is approximately 0.69 mm at the 1/e2radius.

In some embodiments, the beam 116 is focused into the sample 124 using a25 mm focal length, 90-degree off-axis parabola, such as those producedby Janos Technology under part number A8037-175. At focus, the beam canbe approximately 33 μm at the 1/e2 radius in the direction of spatialchirp. The pulse width at focus can be approximately 74 fs full-width athalf maximum (FWHM) amplitude with the assumption of a hyperbolic secantpulse shape.

In some embodiments, the temporally chirped pulses of the input light104 can be spatially chirped then collimated by the gratings of thefirst optical element or set of optical elements 108. After the secondgrating, the intensities, I, of all frequencies of the first outputlight 116 overlap in time, t, but the frequencies do not overlapspatially except at the focal plane of the sample 124.

With reference now to FIG. 2, a non-limiting example application of thesystem 100 will be described in accordance with embodiments of thepresent disclosure. Specifically, a sample system 200 can be set upwhich includes a cutting light input 204 which is focused at a focalpoint 228 on the back plane of a sample 208. The cutting light input 204may correspond to an output of light from the second optical element orset of optical elements 120. In other words, the cutting light input 204may correspond to spatially chirped pulses of light that are temporallyand spatially focusing at the back plane of the sample 208.

The sample 208 may be immersed in a first fluid 220 contained in a firstchamber 212. In some embodiments, the first chamber 212 corresponds toglass or any other type of optically transparent material. The firstchamber 212 may be partially immersed in a second fluid 224 contained inan ultrasonic bath 216.

Experiments have been performed with the system 200 where channelsand/or holes were ablated on the back surface of a 6 mm thick fusedsilica window. Debris removal was aided by ultrasonic waves generated inthe second fluid 224 by the ultrasonic bath 216. The sample 208 wasmounted in a partially immersed glass chamber 212 with 1 mm thick walls.Microfluidic channels were lengthened at a scan rate of 15 μm/s anddeepened by 10 μm steps in the axial direction between scans. Todetermine the maximum aspect ratio of features, holes were machined inthe back surface of the sample 208 by scanning the sample 208 axially at10 μm/s.

An important quality of temporally focused pulses is a symmetric laserspot at the focus of the off-axis parabola. FIG. 3A depicts one exampleof a ray-tracing model where light 304 is focused by a parabolic mirror(or lens) 308 at a focal point 312. The light 304 may correspond to thefirst output light 116. The ray-tracing depicted in FIG. 3A can yield adiffraction-limited, round spot in focus, which indicates that both thespatially chirped and non-spatially chirped dimensions of the beam focusto the same size, as can be seen in FIG. 3B. The symmetry of the focalspot 312 was experimentally verified and is shown in FIG. 3C.Ray-tracing can be performed with 6 mm of fused silica placed before thefocal plane as a means to simulate backside machining. The focal spot isno longer symmetric. Dispersion from the fused silica stretched thefocal spot along the spatially chirped dimension as different colorsfocused to slightly different lateral positions FIG. 3D. The asymmetrywas directly dependent on the thickness of the glass. With theexperimental beam parameters described above, a symmetric,diffraction-limited focal spot can be obtained for fused silica samplesless than approximately 2 mm thick.

While ray-tracing provides details on the geometrical aspects of beampropagation that result from temporal focusing, additional insight intothe behavior of the laser pulses can be gained by calculating thespatio-temporal pattern of a pulse as it propagates through free space.These aspects are depicted in more detail in FIGS. 4A-E and 5. Inparticular, as can be seen in FIG. 4E, the pulse appears as a travelingwave that is transform-limited in time as well as diffraction-limited inspace.

In accordance with at least some embodiments of the present disclosure,the success of temporal focusing for backside micromachining (and othersimilar applications) lies with the improvement in the axial confinementof the intensity of light and the reduction in the out-of-focusnonlinear interaction with the substrate.

As can be seen in FIG. 5, the depth of focus and the nonlinear phaseaccumulation, referred to as the B-integral, are plotted as a functionof the spatial chirp. The degree of spatial chirp is given by the beamaspect ratio (BAR): the ratio of the spatially chirped beam diameter tothe non-spatially chirped beam diameter. The depth of focus is measuredas the half width at half maximum (HWHM) amplitude of the axiallydependent intensity profile. The peak value of the intensity has beenrecorded for each axial position in the simulation depicted by FIGS.4A-E. The HWHM can then be calculated for the full peak-intensityprofile, and this process was repeated for each value of the BAR. It hasbeen observed that the B-integral decreases as the BAR increases (insetto FIG. 5). For example, increasing the BAR from 4 to 8 results in morethan a factor of 10 improvement in the B-integral. Embodiments of thepresent disclosure can produce a BAR of approximately 11.

FIG. 6 shows a comparison of channel or hole profiles created in asample 604 using a number of different ablation processes. A firstchannel profile 608 created by chemical etching is relatively shallow ascompared to a second channel profile 612 created with front surfacelaser ablation (e.g., laser ablation where the laser beam is focused onthe front surface of the sample 604) and a third channel profile 616created using back surface laser ablation concepts of the presentdisclosure.

There are also significant differences between the second channelprofile 612 and third channel profile 616. Specifically, the secondchannel profile 612 is not as deep as the third channel profile 616. Onereason for this is because the laser beam must pass through the debristhat is created during the early stages of ablation. Another reason forthis is due to the shape of the beam and the refraction of the light onthe walls of the channel.

On the other hand, back surface laser ablation as proposed herein iscapable of focusing on the sample 604 without having to travel throughthe debris created during ablation or travel through the channel. Notonly does this enable a deeper channel depth, but the third channelprofile 616 can create a more rectangular profile as compared to the“v-type” profile created by front surface laser ablation.

The ability to machine channels in glass, plastic, and other similaroptically transparent materials with a high aspect ratio (the depth towidth ratio of the channel cross section) may benefit several types ofmicrofluidic applications. For instance, embodiments of the presentdisclosure may be employed to create channels with an aspect ratio of 2sufficient to focus particles into one plane along the depth dimension(approximately 2).

It may also be possible to employ aspects discussed herein to design arapid microfluidic mixer for reaction dynamics studies where rapidmixing is achieved by squeezing reagents through a nozzle with a largeaspect ratio (approximately 10). The dynamics of such a system can bemeasured with confocal microscopy. Even higher aspect ratio channelswould be necessary for optical measurements that have a larger Rayleighrange, as is common for absorption measurements, to achieve mixinguniformity in the axial dimension.

Many types of experiments in microfluidic devices may benefit from theproperties of glass. In other words, it may be beneficial to employprinciples of the present dislcosure to create microfluidic devices fromglass and other silica-based materials rather than creating such devicesfrom the traditional PDMS. The mechanical strength of glass withstandspressure from the high flow rates used in inertial focusing and in rapidmixing. The chemical stability of glass is an advantage for chemicalreaction studies, and the optical transparency of glass benefits whitelight imaging and absorption and fluorescence measurements.

The common fabrication approach for glass microfluidic devices, i.e.,chemical etching, cannot yield the high aspect ratio channels requiredin these studies. Microfluidic channels fabricated by chemical etchinghave aspect ratios of less than one.

With reference now to FIG. 7, a simplified block diagram depicting anoptical system capable of manipulating light (e.g., laser light or alaser beam) will be described in accordance with embodiments of thepresent disclosure. The system depicted in FIG. 7 may represent asimplified version of the system 100 depicted in FIG. 1. The opticalsystem of FIG. 7 comprises a first light input 704, a first opticalelement 708, a first light output 712, a second optical element 716, asecond light output 720, and an object 724. The first light input 704may be similar or identical to the input light 104. Specifically, thefirst light input 704 may correspond to a pulsing laser output. In someembodiments, the first light input 704 may be pulses of a duration onthe order of femtoseconds. The first optical element (or elements) 708may be configured to receive the temporally chirped pulses of the firstlight input 704, spatially chirp and then collimate the frequencies. Theresultant output of the first optical element (or elements) 708 is thefirst light output 712. In some embodiments, a plurality of mirrorsand/or lenses may be provided as the first optical element (or elements)708.

In accordance with embodiments of the present disclosure, the firstlight output 712 may comprise a plurality of spatially separatedwavelengths (or frequencies) of light, but the intensities of eachspatially separated wavelength (or frequencies) may still overlap intime. In other words, the first light output 712 comprises spatiallychirped pulses of light that are collimated and the temporal chirp isremoved. However, the spatially separated wavelengths (or frequencies)do not overlap spatially. Rather they are conditioned to travel parallelto one another until they reach the second optical element 716.

The second optical element 716 may correspond to a focusing element. Insome embodiments, the second optical element 716 comprises a parabolicmirror and/or shaped lens (either concave or convex). In someembodiments, a plurality of mirrors and/or lenses may be provided tospatially focus the first light output 712. Accordingly, the output ofthe second optical element 716 may be considered a focused output.

In accordance with at least some embodiments of the present disclosure,the second optical element 716 spatially focuses the first light output712 to create the second light output 720. The second light output 720may be focused on the object 724 to alter one or more properties of theobject 724. In some embodiments, the second light output 720 may befocused on the back surface of the object 724 to perform back surfacelaser ablation. A more specific application for the second light output720 would be to ablate biological materials. Ablation of biologicalmaterials does not necessarily require back surface ablation, especiallywhere the biological material is not optically transparent. Accordingly,the spatially chirped pulses described herein may also be used in frontsurface ablation without departing from the scope of the presentdisclosure.

Ablation with temporal focusing can potentially enable a variety offemtosecond laser surgeries, which have been previously demonstratedwith standard beams and under nonphysiological, dry conditions. Studiesof neuronal activity typically require thinning or removal of a portionof the skull to provide optical access to the brain. During thisprocess, continual flushing with physiological saline is necessary tomaintain tissue viability and to rinse away debris. Nonlinearinteraction with the saline prohibits the use of low NA beams. However,with spatially chirped pulses and temporal focusing, rapid materialremoval under aqueous immersion is possible at low NA.

Ablation on the backside or in the interior of a transparent biologicaltissue (e.g., biological materials found in the human eye) should alsobe possible with consideration of the optical properties of the tissue,such as the shape and homogeneity, that may be expected to directlyaffect the quality of the focus.

FIG. 8 depicts a possible modification to the system of FIG. 7. Theoptical system depicted in FIG. 8 includes an optical conduit 804 thatis capable carrying the various wavelengths (or frequencies) of thespatially chirped and collimated laser pulses (e.g., the first lightoutput 712) across a distance. It may also be possible to configure theoptical conduit 804 to cause the first light output 712 to travel anon-linear path from the first optical element 708 to the second opticalelement 716. This may be particularly useful to faciliateminimally-invasive surgeries (e.g., arthroscopic surgeries) ornon-invasive procedures.

More specifically, and as can be seen in FIG. 9, the optical conduit 804may comprise a bundle of optical fibers 808, each of which areresponsible for carrying different wavelengths (or frequencies) of thespatially chirped and collimated light (e.g., the first light output712). The optical conduit 804 and the optical fibers 808 may becontained within an endoscopic device. The optical conduit 804 may beresponsible for carrying the first light output 712 into a human body.The second optical element 716 may be positioned at the end of theendoscopic device, thereby allowing the first light output 712 to befocused into the second light output 720. Focusing of the second lightoutput 720 may facilitate the cutting, fusing, etc. of biologicalmaterial within a patient.

Compensation mechanisms may also be included in the optical conduit 804or in the second optical element to account for the differentwavelengths traveling different distances within the optical conduit804. More specifically, a compensation mechanism can be envisioned whichdynamically measures the length (or amount of bend) of each opticalfiber 808 in the optical conduit 804 and delays one or more wavelengths(or frequencies) of light in certain optical fibers 808 to ensure thatall wavelengths (or frequencies) of a spatial chirp are temporallyoverlapped when they reach the second optical element 716. The delayingmechanisms may correspond to any type of known mechanisms for delayinglight propagation, but, if possible, should not introduce substantiallosses.

It should be noted that the cross-sectional shape of the optical conduit804 and/or optical fibers 808 do not necessarily need to be circular.Rather, any arrangement of optical fibers 808 in the optical conduit 804may be accommodated. For instance, the optical fibers 808 may bearranged in a linear array and the optical conduit 804 may have a squareor rectangular cross-sectional profile. As another example, the opticalconduit 804 may comprise an elliptical cross-sectional profile. Otherprofile shapes are also possible.

It has been demonstrated above that the use of temporal focusing withlow numerical aperture beams for femtosecond micromachining can beachieved. This geometry mitigates nonlinear interactions such thatmaterial can be ablated through large path lengths of opticallytransparent material. It has also been shown that features with muchhigher aspect ratios can be produced using this technique as comparedwith features which are produced using chemical etching. Althoughbackside laser ablation is not the only technique for producing highaspect ratio channels in glass, it does add to the already extensivelist of machining capabilities for femtosecond lasers in transparentmaterials and it is the only single-step fabrication method forproducing high aspect ratio channels in glass, which is especiallyuseful for rapid prototyping.

High aspect ratio channels are extremely useful for some types ofmicrofluidic experiments that implement inertial focusing or rapidmixing, for example. With temporal focusing, aspect ratios up to about26 can be achieved. The aspect ratio is not necessarily the same in thespatially chirped and unchirped beam dimensions. This effect isconsistent with simulations that predicted an elongated focal spot inthe chirped dimension as a result of dispersion from a 6 mm thick fusedsilica substrate. This asymmetry is expected to increase for thickersamples, an important consideration for attaining high aspect ratiofeatures in thick substrates. In principle, it is possible toprecompensate for this chromatic aberration.

Machining at low NA results in a large volume of material removed perpulse. Microfluidic channel machining rates of about 25 μm3 per 80 μJpulse have been disclosed herein. However, the low repetition rate ofsome lasers discussed herein is prohibitive for fabricating completemicrofluidic devices that typically consist of centimeters of channels.For example, at the demonstrated machining rate, 100 μm squaremicrofluidic channels could be machined at a rate of one linear cm perhr. Clearly, higher repetition rate lasers would be more useful forfull-scale microfluidic device fabrication, and such lasers arecommercially available. For instance, a 100 kHz laser could machine thechannel at a rate of 1.5 linear cm per min at the same pulse energy.This is an attractive machining rate for centimeter-scale microfluidicdevices. In fact, this machining rate would surpass other methods suchas fabrication by etching and/or lithography for many microfluidicchannel geometries.

Machining at low NA can also improve the ablation rate of bone underaqueous immersion, and the same laser system should allow ablation of acubic millimeter of bone in under one minute. In general, it isanticipated that femtosecond laser micromachining with temporal focusingmay benefit other biomedical applications such as all optical histologyand deep tissue ablation in laser surgery.

Specific details were given in the description to provide a thoroughunderstanding of the embodiments. However, it will be understood by oneof ordinary skill in the art that the embodiments may be practicedwithout these specific details.

While illustrative embodiments of the disclosure have been described indetail herein, it is to be understood that the inventive concepts may beotherwise variously embodied and employed, and that the appended claimsare intended to be construed to include such variations, except aslimited by the prior art.

What is claimed is:
 1. An optical system, comprising: a first opticalelement or set of optical elements that receives temporally chirpedpulses of light and then spatially chirp the received pulses of light;and a second optical element or set of optical elements that focuses thespatially chirped light that is output by the first optical element orset of optical elements.
 2. The optical system of claim 1, wherein thespatially chirped pulses of light are also collimated to overlap in timebut not overlap spatially and wherein the second optical element or setof optical elements focuses the collimated and spatially chirped lightthrough an object onto a back surface of the object.
 3. The opticalsystem of claim 2, wherein the focused light ablates the back surface ofthe object.
 4. The optical system of claim 3, wherein the objectcomprises an optically transparent material.
 5. The optical system ofclaim 4, wherein the object comprises glass.
 6. The optical system ofclaim 4, wherein the object comprises a biological material.
 7. Theoptical system of claim 4, wherein the object is less than 2 mm thickand wherein the focused light creates a substantially uniform focalspot.
 8. The optical system of claim 1, wherein the first opticalelement or set of optical elements comprises a grating system.
 9. Theoptical system of claim 8, wherein the grating system includes twodiffraction gratings that are configured to minimize second and thirdorder dispersion of light as it travels through the first opticalelement or set of optical elements.
 10. The optical system of claim 8,wherein the grating system includes at least one of a grating and prismconfigured to diffract light.
 11. The optical system of claim 1, whereinthe temporally chirped pulses of light comprise femtosecond laserpulses.
 12. The optical system of claim 11, wherein the femtosecondlaser pulses are positively chirped to avoid pulse front tilt prior toreaching the second optical element or set of optical elements.
 13. Theoptical system of claim 1, wherein the second optical element or set ofoptical elements comprises at least one of a lens and parabolic mirror.14. The optical system of claim 1, further comprising an optical conduitwhich includes a plurality of optical fibers, wherein each of theoptical fibers carry different wavelengths of the spatially chirped andcollimated light from the first optical element or set of opticalelements to the second optical element or set of optical elements.
 15. Amethod, comprising: receiving temporally chirped pulses of a light beam;spatially chirping and collimating the temporally chirped pulses of thelight beam such that different frequencies of the spatially chirped andcollimated light overlap in time; and focusing the different frequenciesof the spatially chirped and collimated light at a focal plane, whereinthe different frequencies of the spatially chirped and collimated lightonly spatially overlap at the focal plane.
 16. The method of claim 15,wherein the focal plane coincides with the back plane of an object. 17.The method of claim 16, wherein the back plane of the object is ablatedwith the focused light.
 18. The method of claim 15, wherein thetemporally chirped pulses of light comprise stretched laser pulses andwherein focusing the different frequencies of the spatially chirped andcollimated light at the focal plane create a focused light beam with anaxial intensity profile that breaks a confocal limit of the light beam.19. The method of claim 15, wherein the temporally chirped pulses oflight are focused at a low numerical aperture.
 20. An optical system formodifying properties of an object, the system comprising: a first set ofoptical elements configured to receive a first light input and produce afirst light output, the first light input comprising temporally chirpedpulses of light and the first light output comprising spatially chirpedand collimated light with different frequencies overlapping in time,wherein the first set of optical elements comprises a single-pass,double grating configuration of optical elements which are used tospatially chirp and collimate the first light input; and a secondoptical element configured to receive the spatially chirped andcollimated first light output and produce a second light output whichfocuses the different frequencies of the spatially chirped andcollimated first light output at a focal plane for ablating at least oneof glass and biological materials.
 21. The optical system of claim 20,wherein the second optical element localizes an axial intensity of thespatially chirped and collimated first light output.
 22. The opticalsystem of claim 1, wherein the focused light ablates the front surfaceof the object.